Infrared spectra of isoquinolinium (iso-C9H7NH+) and isoquinolinyl radicals (iso-C9H7NH and 1-, 3-, 4-, 5-, 6-, 7- and 8-iso-HC9H7N) isolated in solid para-hydrogen.

Protonated polycyclic aromatic nitrogen heterocycles (H+PANH) are prospective candidates that may contribute to interstellar unidentified infrared (UIR) emission bands because protonation enhances the relative intensities of the bands near 6.2, 7.7 and 8.6 µm, and the presence of the N atom induces a blue shift of the ring-stretching modes so that the spectra of H+PANH match better with the 6.2 µm feature in class-A UIR spectra. We report the infrared (IR) spectra of protonated isoquinoline (the 2-isoquinolinium cation, iso-C9H7NH+), its neutral counterpart (the 2-isoquinolinyl radical, iso-C9H7NH), and another mono-hydrogenated product (the 6-isoquinolinyl radical, 6-iso-HC9H7N), produced on the electron-bombardment of a mixture of isoquinoline (iso-C9H7N) with excess para-hydrogen (p-H2) during matrix deposition at 3.2 K. To generate additional isomers of hydrogenated isoquinoline, we irradiated iso-C9H7N/Cl2/p-H2 matrices at 365 nm to generate Cl atoms, followed by IR irradiation to generate H atoms via Cl + H2 (v = 1) → HCl + H; the H atoms thus generated reacted with iso-C9H7N. In addition to iso-C9H7NH and 6-iso-HC9H7N observed in the electron-bombardment experiments, we identified six additional hydrogenated isoquinoline species, 1-, 3-, 4-, 5-, 7- and 8-iso-HC9H7N, via their IR spectra; hydrogenation on the N atom and all available carbon atoms except for the two sharing carbon atoms on the fused ring was observed. Spectral groupings were achieved according to their behaviors after maintenance of the matrix in darkness and on secondary photolysis at various wavelengths. The assignments were supported via comparison of the experimental results with the vibrational wavenumbers and IR intensities of possible isomers predicted using the B3LYP/6-311++G(d,p) method. The implications in the identification of the UIR band are discussed.

Spectral Evidence of Bevel-Gear-Type Rotation of Benzene around Br in Solid p-H2: Infrared Spectrum of the C6H6Br Radical.

Whether the structure of C6H6X (X = halogen), an intermediate in the halogenation of benzene, is an open or a bridged form has been debated. We produced Br to react with C6H6 upon photolysis in situ of a Br2/C6H6/p-H2 matrix at 3.2 K. In contrast to the C6H6Cl σ-complex reported previously, the observed infrared spectrum indicates that C6H6Br is an open-form π-complex. Furthermore, lines of the two CH out-of-plane bending modes associated mainly with even- and odd-numbered carbons, predicted near 672 and 719 cm-1, merged into a broad line at 697.3 cm-1, indicating that these modes become nearly equivalent as Br migrates from one carbon atom to another. Quantum-chemical calculations support that the benzene ring performs a bevel-gear-type rotation with respect to Br. Observation of only trans-ortho- and trans-para-C6H6Br2 suggests that this gear-type motion allows the additional Br atom to attack C6H6Br only from the opposite side of the Br atom in C6H6Br.

A chemical link between methylamine and methylene imine and implications for interstellar glycine formation.

Methylamine CH3NH2 is considered to be an important precursor of interstellar amino acid because hydrogen abstraction might lead to the aminomethyl radical •CH2NH2 that can react with •HOCO to form glycine, but direct evidence of the formation and spectral identification of •CH2NH2 remains unreported. We performed the reaction H + CH3NH2 in solid p-H2 at 3.2 K and observed IR spectra of •CH2NH2 and CH2NH upon irradiation and when the matrix was maintained in darkness. Previously unidentified IR spectrum of •CH2NH2 clearly indicates that •CH2NH2 can be formed from the reaction H + CH3NH2 in dark interstellar clouds. The observed dual-cycle mechanism containing two consecutive H-abstraction and two H-addition steps chemically connects CH3NH2 and CH2NH in interstellar media and explains their quasi-equilibrium. Experiments on CD3NH2 produced CD2HNH2, in addition to •CD2NH2 and CD2NH, confirming the occurrence of H addition to •CD2NH2.

Non-energetic, Low-Temperature Formation of Cα-Glycyl Radical, a Potential Interstellar Precursor of Natural Amino Acids.

The reaction of H atoms with glycine was investigated at 3.1 K in para-H2, a quantum-solid host. The reaction was followed by IR spectroscopy, with the spectral analysis aided by quantum chemical computations. Comparison of the experimental IR spectrum with computed anharmonic frequencies and intensities proved that, regardless of the reactant glycine conformation, Cα-glycyl radical is formed in an H-atom-abstraction process with great selectivity. The product of the second H-atom abstraction, iminoacetic acid, was also observed in a smaller amount. The Cα-glycyl radical is sensitive to UV light and decomposes to iminoacetic acid and H atom upon 280 nm radiation. Since the reactive radical center is located on the Cα-atom, it is suggested that natural α-amino acids can be formed from glycine via the Cα-glycyl radical by non-energetic mechanisms in the solid phase of the interstellar medium.

Interplay between hydrogen bonding and n→π* interaction in an analgesic drug salicin.

The competition and cooperation between weak intermolecular interactions are important in determining the conformational preferences of molecules. Understanding the relative strengths of these effects in the context of potential drug candidates is therefore essential. We use a combination of gas-phase spectroscopy and quantum-chemical calculations to elucidate the nature of such interactions for the analgesic salicin [2-(hydroxymethyl)phenyl ß-d-glucopyranoside], an analog of aspirin found in willow bark. Of several possible conformers, only three are observed experimentally, and these are found to correspond with the three lowest energy conformers obtained from density functional theory calculations and simulated Franck-Condon spectra. Natural bond orbital analyses show that these are characterized by a subtle interplay between weak n→π* interaction and conventional strong hydrogen bond, with additional insights into this interaction provided by analysis of quantum theory of atoms in molecules and symmetry-adapted perturbation theory calculations. In contrast, the higher energy conformers, which are not observed experimentally, are mostly stabilized by the hydrogen bond with negligible contribution of n→π* interaction. The n→π* interaction results in a preference for the benzyl alcohol group of salicin to adopt a gauche conformation, a characteristic also found when salicin is bound to the ß-glucosidase enzyme. As such, understanding the interplay between these weak interactions has significance in the rationalization of protein structures.

Evidence for phosphorus bonding in phosphorus trichloride-methanol adduct: a matrix isolation infrared and ab initio computational study.

The weak interaction between PCl3 and CH3OH was investigated using matrix isolation infrared spectroscopy and ab initio computations. In a nitrogen matrix at low temperature, the noncovalent adduct was generated and characterized using Fourier transform infrared spectroscopy. Computations were performed at B3LYP/6-311++G(d,p), B3LYP/aug-cc-pVDZ, and MP2/6-311++G(d,p) levels of theory to optimize the possible geometries of PCl3-CH3OH adducts. Computations revealed two minima on the potential energy surface, of which, the global minimum is stabilized by a noncovalent P···O interaction, known as a pnictogen bonding (phosphorus bonding or P-bonding). The local minimum corresponded to a cyclic adduct, stabilized by the conventional hydrogen bonding (Cl···H-O and Cl···H-C interactions). Experimentally, 1:1 P-bonded PCl3-CH3OH adduct in nitrogen matrix was identified, where shifts in the P-Cl modes of PCl3, O-C, and O-H modes of CH3OH submolecules were observed. The observed vibrational frequencies of the P-bonded adduct in a nitrogen matrix agreed well with the computed frequencies. Furthermore, computations also predicted that the P-bonded adduct is stronger than H-bonded adduct by ∼1.56 kcal/mol. Atoms in molecules and natural bond orbital analyses were performed to understand the nature of interactions and effect of charge transfer interaction on the stability of the adducts.

Reactivity between non-energetic hydroxyl (OH) radicals and methane (CH4).

Reactions between dilute methane and nonenergetic hydroxyl radicals were carried out at 3.5 K. The temperature was kept low in order to characterize the stepwise reaction and prevent parasitic side reactions. The hydroxyl radicals originate from discharged H(2)O/He mixtures. The reactions were monitored in situ using a Fourier transform infrared spectrometer. The formation of CH(3) radicals was confirmed simultaneously with the formation of water ice. Subsequent recombination reactions lead to the formation of ethane (C(2)H(6)). Production of ethane and water ice occur preferentially to the formation of methanol.